Geographical and temporal patterns of lemming population dynamics in Fennoscandia

There is a long-lasting debate in ecology on cyclicity. synchrony and time lags of lemming population fluetuations. We have analysed 137 yr of previously published population data on the Norwegian lemming Lemmus lemmus in ten geographic regions of Fennoscandia. The dominating pattern was synchronous 4-yr cycles. There was no support for the hypothesis of a north-south gradient in cycle length. However, we found periods of prolonged interruptions in the cyclicity, which were more eommon in northern areas. We found a high degree of synchrony between regions, with only a weak relationship to distance. The observed pattern in lemming population dynamics was more consistent with effects from extrinsic factors, such as climate, than intrinsic factors, such as dispersal.